Geological controls and methane sorption capacity of marine shales of the Fuling shale gas field in the eastern Sichuan Basin, China

Abstract

High-pressure methane sorption isotherms were performed on a series of Upper Ordovician Wufeng and Lower Silurian Longmaxi shale samples in the Fuling shale gas field to investigate the effects of organic matter content, thermal maturity, clay minerals, pore structure, temperature and pressure on methane sorption capacity. A large number of micropores with a pore width of less than 10 nm are developed within the organic matter, with its abundant specific surface areas as the fundamental factor to enhance the methane sorption capacity. The total organic carbon (TOC)-normalized sorption capacity increases with an increasing equivalent vitrinite reflectance R o , but an opposite trend is observed when R o is in the highly over-mature stage. The TOC-normalized sorption capacity shows no correlation with the total clay content as well as individual clay minerals. Most of the excess sorption capacity of shales increases with an increasing pressure, exhibits a maxima in the pressure range of 15–17 MPa and then decreases. The sorption isotherms show an obvious decrease in excess sorption capacity with increasing temperature. Moreover, the Langmuir pressure exponentially decreases with the reciprocal of temperature. Based on the Langmuir adsorption model, an empirical formula is established to evaluate the absolute sorption capacity of shales as a function of TOC content, pressure and temperature. Supplementary material: Additional description of experimental data are available at https://doi.org/10.6084/m9.figshare.c.3714040.v2

Comparison and Significance between Low- and Normal-Resistivity Shales of Wufeng-Longmaxi Formation in Changning Area, Southern Sichuan Basin

“…Organic matter is the material basis for oil and gas generation, and its content is an important factor in controlling the enrichment of shale gas. Second, after a certain degree of thermal evolution of organic matter, abundant organic nanopores can be formed during hydrocarbon generation and expulsion, providing storage space for the occurrence of shale gas. − The maturity of organic matter is a key factor affecting the evolution of hydrocarbon generation, which not only characterizes different hydrocarbon generation stages and potentials, but also has a certain controlling effect on reservoir properties. − Meanwhile, an excessively high degree of thermal evolution leads to the graphitization of organic matter, collapse and filling of organic pores, a significant increase in the crystallinity of clay minerals, and a decrease in the volume of intergranular pores, resulting in a certain reduction in both the organic pores and intergranular pores of clay minerals . Both the Wufeng and Longmaxi Formations in Changning area have high organic matter abundance, and the TOC content is mainly 1−6% (Figure ).…”